The present invention pertains to an air/fuel ratio control for an internal combustion engine using an exhaust gas sensor, and in the preferred embodiment disclosed herein is concerned with an electronic feedback carburetor system including an oxygen sensor.
Although the basic concepts relating to air/fuel ratio control systems for automotive internal combustion engines using exhaust gas sensors have been long known, in recent years there have been a number of patents issued relating to improvements in such systems. Generally, the improvements are a result of the application of electronic technology to the problem of reducing exhaust emissions output of the engine while improving the engine fuel economy and obtaining satisfactory driveability of the vehicle. Some of these improvements are relatively crude and unsophisticated. Others are more elaborate and complicated.
In addressing the problem of designing an electronic feedback carburetor system applicants have made new discoveries and have developed a new and unique system which achieves new and unique modes of operation resulting in significant improvements in a number of different respects over other systems of which applicants are aware. As a result, an electronic feedback carburetor system embodying principles of the invention attains heretofore unachieved results and exhibits advantages which are not provided by other systems. Moreover, the invention, in its preferred embodiment, makes use of the latest electronic technology to provide a system wherein the electronics can be conveniently and economically packaged for mass production usage, yet is capable of being readily programmed to meet specific engine requirements. While details of the invention will be explained later in the description of the preferred embodiment, the more impressive improvements which are believed new and unique in applicants' system may be generally set forth as follows.
One feature of the present invention relates to the development of a control signal which provides for more precise regulation of the air/fuel ratio when the system is operating in the closed-loop mode. One problem in obtaining precision control arises from the limitations of commercially available oxygen sensors which are suitable for use in an automotive vehicle. These sensors present an impediment because they only possess a switching characteristic at stoichiometry and can therefore indicate only a rich mixture or a lean mixture condition. Applicants have overcome this impediment through the provision of an integrator circuit and a stability circuit which both receive a rectangular waveform signal derived from the oxygen sensor. The two circuits in turn develop respective output signals which cooperate to produce a composite signal which controls the air/fuel ratio. The integrator by itself develops a ramp type signal which ramps in one direction when the oxygen sensor is in one state and in the opposite direction when the oxygen sensor is in the other state. The stability circuit is responsive to transitions of the oxygen sensor from one state to the other and develops a signal which may be generally described as being the derivative of the oxygen sensor signal. This composite signal referred to above is developed by algebraically summing the integrator and stability circuit signals. In response to a change in state in the oxygen sensor, this composite signal commands a predetermined amount of correction of the air/fuel ratio which is maintained at essentially a constant level for a time interval essentially equal to the transport time of the mixture from the carburetor through the engine to the oxygen sensor. With the engine operating at a reasonably steady state condition, the amount of correction is such that by the conclusion of the transport time interval, the oxygen sensor will have switched back to its original state. In this way, the air/fuel ratio is closely regulated to be within a narrow window about the desired operating point which may be at or in the vicinity of stoichiometry. This enables a more precise and accurate control of the air/fuel ratio to be obtained which is advantageous in securing the best performance of certain types of catalysts which subsequently treat the exhaust gases after they have passed by the oxygen sensor. While the disclosed embodiment utilizes analog circuits, it will be appreciated that the principles of this aspect of the invention may be applied to other embodiments using digital circuits or microprocessors. Where the engine is operating under a more dynamic condition and the amount of correction is insufficient to change the state of the oxygen sensor, additional correction is performed.
Another feature of the invention is that there are additional circuits which are responsive to more extreme transient conditions, such as substantial changes in engine load, engine deceleration, etc., and are operative to interrupt the closed-loop mode of operation in favor of an open-loop mode of operation.
A further feature of the invention is that when the closed-loop mode of operation is interrupted, the output signal of the integrator circuit is locked (or held in memory) so that when the closed-loop mode of operation resumes, the integrator output signal is at a level which will enable the system to quickly return to the window about the desired operating point.
Still another feature of the invention relates to the provision of a programming device in the circuit whereby the closed-loop operating point may be programmed without having to make changes in the layout of the circuit board containing the circuit electronics. According to this aspect of the invention a programming circuit section which is associated with the integrator contains a socket which is hard-wired onto the circuit board. Another element, called a header, is inserted into the socket to perform the programming function. The header contains circuit paths which connect certain of the terminal pins on the socket with certain other terminal pins in such a way that a selected characteristic is programmed into the circuit depending upon the particular header which is used. This is of significant advantage in the application of the invention to the mass production of automotive engines since it means that changes in the calibration of the system can be made expeditiously and without requiring substantial tooling changes. Thus, rather than having to change components on the circuit board and the circuit board layout, all that is necessary is to make a new header which can be done expediently and without any substantial amount of tooling change. Circuitry on the board coacts with the programming device to shift the operating point under certain conditions of engine operation, and this constitutes a further feature of the invention.
The system also includes circuits responsive to initial operating conditions of the engine whereby the closed-loop mode of operation is prevented until both the engine is warmed up and a certain "after start" timing interval has elapsed after the engine has started. During this initial open-loop mode of operation, an analog coolant temperature signal related to engine temperature is utilized to control the air/fuel mixture to the exclusion of the composite signal from the integrator and stability circuits.
Another aspect of the invention provides for detection of certain system faults or failures. For example, the disclosed embodiment has a fault detection circuit which is particularly useful in connection with detection of a failed oxygen sensor. When such a failure is detected, a fault signal is given to both provide an alarm via an alarm circuit and is also utilized to control the air/fuel ratio to the exclusion of the other signals which usually control the air/fuel ratio.
Additional features are also disclosed and may be seen with reference to the ensuing disclosure and accompanying drawings. Naturally, the recitation of the inventive features set forth above is merely to acquaint the reader with the disclosure and should not be construed as limiting the scope of the invention or its various aspects because it is the set of claims at the conclusion of this specification which define the invention in its various aspects.
The invention is disclosed in connection with a preferred embodiment thereof according to the best mode presently contemplated in carrying out the invention.